1. Technical Field
The present invention relates to a process and apparatus for treating molten aluminum to add hydrogen gas to satisfy the affinity of molten aluminum for hydrogen and to improve its casting properties.
2. Background Information
The term "aluminum" as used in the subject application includes pure aluminum metal and all aluminum alloys whether or not aluminum constitutes the major constituent of such metal alloys.
Molten aluminum prior to casting contains dissolved hydrogen gas in widely varying amounts along with other non-metallic inclusions such as aluminum oxide and magnesium oxide which are normally considered undesirable impurities. It has been considered that hydrogen gas, when present in molten aluminum, may produce defects in ingots prepared from the melt and also in products manufactured from such ingots. It is heretofore been thought that hydrogen gas must be removed from the molten metal along with non-metallic inclusions in the manufacture of high-quality aluminum products.
In the prior art, hydrogen gas and non-metallic inclusions are frequently removed from molten aluminum by introducing an inert gas into the molten metal in the form of bubbles. Normally, since the atmosphere over the molten aluminum contains some water, it has been thought that the water in the atmosphere over the surface of the molten aluminum metal presents problems when the hydrogen from the disassociated water penetrates into the melt. The surface of molten aluminum which is usually quiescent during the melting is normally covered with a thin aluminum oxide coating or layer so that the water in the overlying atmosphere does not normally react with aluminum. However, when a treating gas such as an inert gas or chlorine gas is forced into the molten aluminum, the bubbles released to float on the surface of the melt break up the surface layer and the overlying aluminum oxide coating exposing the melt to the atmosphere at the broken surface areas. The water in the atmosphere then reacts with aluminum before the oxide coating is allowed to reform, hydrogen gas being produced from disassociation of the water and then penetrating into the melt in an uncontrolled fashion.
Processes have been proposed in which the enclosed treating vessel for containing molten aluminum is filled above the surface of the molten aluminum with a treating gas at a higher pressure than atmospheric pressure such as disclosed in U.S. Pat. No. 3,870,511.
The process disclosed therein requires a large volume of inert treating gas involving a relatively costly treating process. Another process for removing hydrogen gas and non-metallic inclusions from molten aluminum is disclosed in U.S. Pat. No. 4,772,319 wherein the water content of the atmosphere above the surface of the molten aluminum is reduced in atmospheric pressure to decrease the amount of hydrogen gas resulting from the reaction between the molten aluminum and the water, the reduced atmospheric pressure achieving hydrogen gas removal from the melt. The atmosphere of the air overlying the melt has a lower dew point than that of the normal atmosphere, inert treating gas being required for hydrogen gas elimination. Such process is not particularly useful in a continuous melting process, however, when raw aluminum ingots are fed continuously or intermittently into the receiving end of a melting furnace and molten aluminum is essentially continuously withdrawn in near equal amounts for casting operations. Maintaining the reduced atmospheric pressure within the furnace requires a costly air-tight furnace construction and air locks difficult to maintain and expensive inert gases to control such melting processes.
U.S. Pat. No. 4,521,001 discloses an apparatus for removing gases from molten aluminum, the furnace being provided with a removable specially-designed cover to maintain a partial vacuum between the cover and the molten metal surface which process is inapplicable to a continuous melting furnace wherein large quantities of molten aluminum are required for essentially continuous casting operations. Such furnaces are normally capable of melting large volumes of molten aluminum such as 20 tons in the manufacture of large automotive engine parts.
Virtually all of the prior art involves the removal of uncontrolled hydrogen gas from the molten aluminum to obtain desired casting properties in the resultant cast articles. None of such art involves the positive introduction of a controlled near saturation level of hydrogen gas into the molten aluminum to obtain improved casting properties in the manufacture of large automotive parts, for example.